Chief Howard Jordan said One Goh, 43, had been interviewed by police and "does not appear remorseful at all," CNN reported Tuesday.
Goh, who was expected to be charged Wednesday, is a former nursing student at Oikos University who had been expelled and was upset with an administrator and students. The reputed gunman said he had been "picked on" and "wasn't treated fairly," Jordan said.
Jordan was unsure why Goh was expelled.
"We've been told that some of the possibilities are that he was expelled for his behavioral problems, anger management, but nothing specific," Jordan said.
Jordan did not identify the administrator Goh complained about, but said she was not injured in the shooting, the Oakland Tribune reported.
"We don't believe that any of the victims were the ones that teased him."
Police Tuesday confirmed the identities of five of those who died in the worst mass killing in almost 20 years in the Bay Area, the Tribune said.
Four women killed were identified as Lydia Sim, 21, of Hayward; Katleen Ping, 24, of Oakland; Sonam Chodon, 33, of El Cerrito; and Grace Eunhae Kim, 23, of Fremont. A man who was killed was identified as Tshering Rinzing Bhutia, 38, of San Francisco.
The gunman took Bhutia's keys and drove off in his car.
Jordan said Goh entered the university Monday morning, took a secretary hostage and went looking for a particular female administrator, who was not there. When Goh realized the administrator was not there, he fatally shot the secretary, police said.
Jordan said investigators do not know whether Goh targeted women. The victims ranged in age from 21-40 and were from countries including Korea, Nepal, Nigeria and the Philippines, Jordan said.
Jordan said Goh did not resist when arrested and was "very cooperative, very matter-of-fact, very calm" and that he remembered specific details about the shootings.
Jordan said the victims had been killed execution-style.
Goh entered a classroom, told the students to line up against a wall, then shot them, Jordan said.
"This was a calculated, cold-blooded execution in the classroom," he said.
Once done, the gunman left the classroom, reloaded his semiautomatic weapon and returned, firing more rounds when he realized other people were hiding in an adjacent classroom, Jordan said.
He then drove away in a victim's car, police said.
"This happened within minutes," Jordan told CNN Tuesday. "We don't think the victims had any opportunity to resist; any opportunity to surrender."
Police said Goh was arrested at Alameda's South Shore Center soon after the massacre at Oikos University. He is a native of Korea and a naturalized U.S. citizen with no criminal record.
Police said Goh purchased a gun legally six weeks ago and they believe it is the gun used in Monday's rampage, although it has not been recovered.
The college caters to the Korean-American Christian community, offering areas of study in theology, music, nursing and Asian medicine.
Lucas Garcia, who was teaching an English class when the shooting began, told KGO-TV, San Francisco, he counted six gunshots ringing out from a nursing classroom.
Garcia said he heard someone yell, "He's got a gun," and he hustled his students out of the building.
A memorial service for the victims was scheduled Tuesday evening.
Larry Reid, Oakland's City Council president who represents the area where the college is located, said he was unaware of the school's existence until Monday, the Los Angeles Times said.
"Today there are a number of individuals who lost their lives to senseless violence," Reid said. "It's just another sad day in my city."
California Gov. Jerry Brown, Oakland's mayor from 1999 to 2007, called the killings "shocking and sad."
"Our thoughts and prayers go out to the victims, their families and friends and the entire community affected by this senseless act of violence," Brown said in a statement.
Citing court records, the Tribune said there have been several court judgments and tax liens against Goh, including a $23,000 federal tax liability at one point, and thousands more in debt to banks and apartment landlords.